The use of switched mode output stage(s) in audio amplification, in which the conversion is performed digitally, allows audio output from digital signal input. The digital signal conversion may be achieved by a variety of devices. One particular technique utilizes a delta-sigma (ΔΣ) modulator, in which the digital input signal is filtered and then quantized to produce an output. In one switched technique, the noise-shaped quantized signal from the quantizer is coupled to a pulsewidth modulator (PWM), in which a value of the quantized signal controls the duty cycle of the PWM output. This PWM output then controls the operation of a switching device or devices, utilized as the output stage of an amplifier to drive a load. The filtered digital signal is used to modulate the pulsewidth of the PWM signal to control the duration of the switch on/off time of the output stage to the load. Thus, delta-sigma modulator circuitry coupled with a PWM circuitry may be utilized to operate as a switching amplifier to drive a load. Accordingly digital signals are converted to an audio drive signal to drive a load, such as an audio speaker.
In a variety of systems employing a switching amplifier, the power supply that sources the voltage to drive the load, may not source a constant supply voltage over the full range of operation of the switching amplifier. For example, in some instances, the power supply may be under-rated for the required operational specification (under-specified). For example, a particular power supply may be specified to provide a given maximum or nominal power to one output channel. However, the same power supply may not be capable of providing the same specified power to multiple channels at the same time or to a single channel over a longer period of time. In other instances, the load may change causing the power supply to operate at less than the fully-rated voltage.
Typically, voltage variations in a power supply, whether due to external factors or changes in circuit operation, are more noticeable with consumer electronic devices as compared to professional units. Since many power supplies in audio equipment are generally under-rated for maximum desired capability, a variety of power supply compensation techniques are employed to regulate the power supply. In a switching amplifier, such as in the ΔΣ/PWM amplifier mentioned above, mechanisms are employed to ensure a fairly constant supply voltage from the power supply. In one technique, the power supply voltage is monitored within the ΔΣ stage to adjust the pulse width of the PWM signal corresponding to changes in the power supply voltage. Therefore, when the power supply voltage changes its value, a corresponding compensating adjustment is made to the pulsewidth of the PWM signal coupled to the output switching device or devices.
Although this compensating technique adjusts for non-constant power supply voltages under slow changing conditions, the technique may fail to compensate if the power supply is under-rated or if certain operating parameter of the power supply is exceeded. For example, the power supply may provide x volts as its nominal output when no load of the amplifier exists. At full load of the amplifier, the supply in this particular example decreases to x-y volts. Therefore, digital full-scale input may be set to the full “rail” voltage of the power supply at x volts or may be set to a lower voltage of x-z volts. If the digital full-scale input is set to the nominal value of the power supply, certain high driving inputs will cause the output to be clipped when the supply voltage decreases. The clipping of the output results in distortion. Alternately, if the full-scale input is set to the lower voltage (x-z), then the output signal will not be driven close to the full capability of the power supply under normal conditions. Although distortion may be reduced, the output is not driven to its full nominal capability and generally the sound is much quieter.
A need exists to have a switching amplifier, which may be driven at or near its nominal supply potential but also limit the amount of the distortion when the supply is driven beyond its capability.